1. Field of the Invention
The present invention relates generally to combustion gas turbine engines and, more particularly, to combustion gas turbine engines that employ catalytic combustion principles in the environment of a lean premix burner.
2. Related Art
As is known in the relevant art, combustion gas turbine engines typically include a compressor section, a combustor section and a turbine section. Large quantities of air or other gases are compressed in the compressor section and are delivered to the combustor section. The pressurized air in the combustor section is then mixed with fuel and combusted. The combustion gases flow out of the combustor section and into the turbine section where the combustion gases power a turbine and thereafter exit the engine. Commonly, the turbine section includes a shaft that drives the compressor section, and the energy of the combustion gases is greater than that required to run the compressor section. As such, the excess energy is taken directly from the turbine/compressor shaft to typically drive an electrical generator or may be employed in the form of thrust, depending upon the specific application and the nature of the engine.
As is further known in the relevant art, some combustion gas turbine engines employ a lean premix burner that mixes excess quantities of air with the fuel to result in an extremely lean-burn mixture. Such a lean-burn mixture, when combusted, beneficially results in the reduced production of nitrogen oxides (NOx), which is desirable in order to comply with applicable emission regulations, as well as for other reasons.
The combustion of such lean mixtures can, however, be somewhat unstable and thus catalytic combustion principles have been applied to such lean combustion systems to stabilize the combustion process. Catalytic combustion techniques typically involve preheating a mixture of fuel and air and flowing the preheated mixture over a catalytic material that may be in the form of a noble metal such as platinum, palladium, rhodium, iridium or the like. When the fuel/air mixture physically contacts the catalyst, the fuel/air mixture spontaneously begins to combust. Such combustion raises the temperature of the fuel/air mixture, which in turn enhances the stability of the combustion process. The requirement to preheat the fuel/air mixture to improve the stability of the catalytic process reduces the efficiency of the operation. A more recent improvement splits the compressed air that ultimately contributes to the lean-burn mixture into two components; mixing approximately 10-20% with the fuel that passes over the catalyst while the remainder of the compressed air passes through a cooling duct, which supports the catalyst on its exterior wall. The rich fuel/air mixture burns at a much higher temperature upon interaction with the catalyst and the coolant air flowing through the duct functions to cool the catalyst to prevent its degradation. Approximately 20% of the fuel is burned in the catalytic stage and the fuel-rich air mixture is combined with the cooling gas just downstream of the catalytic stage and ignited in a second stage to complete combustion and form the working gas for the turbine section.
In previous catalytic combustion systems, the catalytic materials typically were applied to the outer surface of a ceramic substrate to form a catalytic body. The catalytic body was then mounted within the combustor section of the combustion gas turbine engine. Ceramic materials were often selected for the substrate in as much as the operating temperature of a combustor section typically can reach 1327° C. (2420° F.), and ceramics were considered as the best substrate for use in such a hostile environment, based on considerations of cost, effectiveness and other considerations. In some instances, the ceramic substrate was in the form of a ceramic wash coat applied to an underlying metal substrate, the catalyst being applied to the ceramic wash coat.
The use of such ceramic substrates for the application of catalytic materials has not, however, been without limitation. When exposed to typical process temperatures within the combustor section, the ceramic wash coat can be subjected to spalling and/or cracking due to poor adhesion of the ceramic wash coat to the underlying metal substrate and/or mismatch in the coefficients of thermal expansion of the two materials. Such failure of the ceramic wash coat subsequently reduces catalytic performance. It is thus desired to provide an improved catalytic body that substantially reduces or eliminates the potential for reduced catalytic performance due to use of ceramic materials.
In certain lean premix burner systems, such as the two-stage catalytic combustors described above, oxidation of the advanced nickel-based alloys, such as Haynes 230 and Haynes 214 commonly employed as the substrate for the ceramic wash coat, at temperatures of 900° C. (1650° F.), not only lead to the formation of either chromia- or alumina-enriched external oxide layer, but also to internal oxidation of the metal substrate. With time, the unaffected cross-sectional wall thickness area of the catalytic combustion substrate tubes decreases and gives rise to a potential reduction in the ultimate load-bearing capabilities of the substrate tube. It is thus desired that an improved catalytic body be provided, that can be used in conjunction with such a multistage combustor section without exhibiting such oxide degradation.